Conventional organic solvent-based industrial finishes and coatings have presented many problems to date. For example, organic solvents can pollute the workplace and environment; many are readily ignitable; some are toxic; and many are quite expensive. Some of these conventional organic solvents, that are added to a coating or finish composition, can lower the quality of the finish. They also can, at times, undesirably add color to an otherwise clear finish.
As a replacement for conventional organic solvent-based finishes, one of the trends in the polymer industry has been toward producing so-called "high solids", solvent-borne coating compositions. Yet another trend in the polymer industry has been toward producing water-based coatings.
While water-based coatings are often preferred, from an environmental standpoint, the use of water-based coatings is not always preferred from an applications or end-use standpoint. For example, the maximum "solids" content of many water-based coatings is about 50-55 wt.-% solids, based on total weight of the composition; and a particular application or end-use might prefer a higher wt.-% solids content.
Second, most water-based coatings require that certain other ingredients (such as surfactants, emulsifiers, and the like) be added to the coating composition formulation, to enable the water-based ingredients to be compatible with the organic-based ingredients, for the coating composition. The result of including the above-mentioned additional ingredients, that are required from an ingredient-compatibility standpoint, is that the overall coating properties are often undesirably affected. For example, most conventional water-based coatings are unable to achieve certain high-gloss levels, achievable by many conventional solvent-based coatings. Depending upon a particular application or end-use, therefore, use of a water-based coating may be particularly undesirable.
Third, depending, of course, upon the application or end-use, the 50-55 wt.-% solids level of the water-based coatings might be unattractive for economic or other business reasons, rendering the relatively-higher wt.-% organic-based coatings the coatings-of-choice.
The high-solids, solvent-borne coating compositions typically have a solids content of about 70 wt.-% (non-volatiles), based upon total weight of the coating composition. The present invention is concerned with high solids, solvent-borne coating compositions of this type.
High solids solvent-borne coating compositions provide significant advantages over conventional, organic solvent-thinned coating compositions. For example, high solids solvent-borne coating compositions do not pollute the air as much as the solvent-thinned coating compositions (which include significantly more solvent). Rather, the high-solids solvent-borne coating compositions have been found to significantly reduce discharge of undesirable solvent fumes, when being applied as a film or coating to a substrate.
It has been demonstrated, moreover, that the preparation of these high-solids solvent-borne coating compositions results in generally reduced energy requirements (for their preparation) in terms of material, energy expended and labor that is required, as compared to conventional organic solvent-thinned industrial finishes and other coating systems that are organic solvent thinned.
Furthermore, unlike organic solvent-thinned systems, the high-solids solvent-borne coating compositions generally present less of a fire hazard as well as less of a toxicity problem.
It has also been demonstrated that high solids solvent-borne coating compositions can provide substantial advantages over certain other conventional coatings systems. Such conventional coating systems include the so-called "solventless" coatings systems, the so-called "water-borne" systems, certain powder systems, and the so-called "non-aqueous" dispersion systems. For example, in certain applications, the high-solids solvent-borne coating compositions have been shown to provide a better overall balance of properties over the conventional coating systems briefly mentioned above.
Another major problem that the coatings industry is faced with at present, moreover, is the manufacture of relatively high solids coating compositions which have a low viscosity. Generally, the trade-off for making high solids coating compositions (which is the desired result in certain applications or end-uses) has been to provide a more viscous coating composition (which is undesired, yet generally necessary, to achieve the desired result). Of course, organic solvent can be added to reduce viscosity; but current government regulations generally severely limit the extent to which such solvent addition is permissible.
A high viscosity coating composition is disadvantageous, as can be appreciated. The disadvantage that results from use of a highly viscous composition is particularly noticeable in the manufacture of coating compositions having so-called "after-market" applications or end-uses. For example, coatings for after-market applications generally require, among other things, low viscosity, rapid air-dry time, and extended pot-life. Low viscosity is desired so as to obtain the particular set of flow properties that are desired. The term "extended pot-life" as used herein means that the coating composition ingredients, after being mixed or otherwise combined, do not gel or otherwise set-up until after about 3 to about 8 hours after being so combined, thereby enabling about 3 to about 8 hours to pass between the time that the composition ingredients are combined and the time that the composition is applied to a substrate. The relative term "rapid air-dry time" is generally understood to mean that procedure whereby the coating composition becomes dry, virtually at the time that the composition is applied to a substrate, resulting in a substantially tack-free and dust-free finish virtually at the time of application.
Thus, a particularly significant problem that the coatings industry is currently addressing is the problem of how to manufacture certain high-solids coating compositions which are able to provide superior coating finishes, yet at the same time, meet government environmental regulations. For example, commercially-acceptable after-market air-dryable coating compositions must possess, among other qualities, a low viscosity, an extended pot-life, and a rapid air-dry time at ambient temperatures. It can be appreciated that ambient temperatures can greatly be affected by locale and season. Furthermore, such coating compositions must provide good weathering capabilities, as measured by gloss-retention after application. Still further, such coating compositions must possess sufficient hardness for their particular application or end-use.
Additionally, such coatings compositions must be relatively high-solids, i.e. at least up to a specified "minimum solids" content, so as to meet government regulations. That is, the coating compositions must, for example, meet certain governmentally-imposed compositional requirements or regulations, one such regulation being generally referred to in connection with the amount of Volatile Organic Compounds (VOCs) that are present in the coating composition. In particular, the VOC requirement defines the acceptable limits of volatile materials allowed in such coating composition.
Until the discovery of our invention, however, it had not been thought possible to be able to provide superior, economically-feasible air-dryable coating compositions having a good balance of the above-mentioned commercially-acceptable attributes yet which is also able to meet governmental regulations as well.
Polyols of acrylic copolymers as well as acrylic polyol copolymer blends for preparing high-solids coating compositions are well-known in the art. Such prior-art polyols, however, while useful in certain applications (e.g. high temperature-cured or so-called "stoved" coatings end-uses or applications), have generally not performed well from an overall performance standpoint, particularly where the end-use or application demands that the coating composition be air-dryable, possess high solids content, and demonstrate reduced viscosity and extended pot-life.
As used herein, the term "stoved" means, after an application of an effective or sufficient amount of heat, that a particular solvent is driven off without the coating composition becoming heat-cured.
U.S. Pat. Nos. 4,529,787 (to Schmidt et al.) and 4,546,160 (to Brand et al.), both patents having been assigned to S. C. Johnson and Son, Inc., of Racine, Wis. (the assignee of this application), disclose processes for or methods of making acrylic copolymers that are useful in preparing coating compositions having high solids, low molecular weight, narrow molecular weight distribution, and controlled viscosity. These acrylic copolymers, sometimes referred to as "stand-alone" acrylic copolymers, have generally been found to be extremely useful in connection with the production of certain coatings, for a variety of applications. Yet, in certain other applications, such as those end-uses or applications requiring that a particular coating composition be air-dryable, there is currently a need to provide a polyol which will enable the coating composition to have acceptably low viscosity, extended pot-life, and comparable-to-conventional or improved weathering capabilities. Typically, gloss-retention and hardness values are measured to determine whether the coating composition will be acceptable to a consumer or other end-user.
The prior art teaches the use of acrylic copolymer blends, for use in connection with coating compositions. In particular, U.S. Pat. No. 4,565,730 (to Poth et al.) discloses a wet-on-wet method for applying a clear lacquer coating composition over a pigment-containing base coat for use in heat-cured or stoved original-equipment manufacture (OEM) end-uses or applications.
The '730 patent, still more particularly, discloses a so-called "binder" consisting essentially of two acrylic polymers and a polyisocyanate. The first acrylic polymer has a molecular weight of 800 to 4000, a hydroxyl number of 80 to 180, and a Tg (i.e., glass-transition temperature) below -10.degree. C. The first acrylic polymer is prepared from 75% to 100% by weight of acrylic acid esters and 0% to 25% by weight of methyacrylic acid esters. The second acrylic polymer has a molecular weight of 3,000 to 10,000, a hydroxyl number of 40 to 120, and a Tg of -10.degree. C. to +10.degree. C. The second acrylic polymer is prepared from 0% to 30% by weight of acrylic acid esters and 70% to 90% by weight of methacrylic acid esters
The composition that is disclosed in the '730 patent differs substantially from that which is disclosed herein because, among other things, the first and second acrylic copolymers of the present invention require, in contradistinction to what is disclosed in the '730 patent, different glass transition temperatures, different molecular weights, different proportions of the copolymers that are utilized, and different proportions of the monomers that are utilized to make up the copolymers disclosed herein.
Furthermore, not disclosed--or even suggested--in the '730 patent are certain features or advantages of the present invention, which will be discussed presently. For example, as one such feature or advantage, the acrylic copolymers of the present invention include, among other things, polyol blends having a low polydispersity value. Those skilled in the art can appreciate the desirability of a low polydispersity value, particularly for certain end-uses or applications.
Another feature or advantage of the present invention is that the acrylic copolymer and polyol blends disclosed herein comprise a low Tg copolymer and a high Tg copolymer, having different hydroxyl numbers. Additionally, the polyol blends of the present invention have a solids content which is relatively greater than that disclosed in the '730 patent, which is highly desirable for certain end-uses.
Additional patents, known in the art, which disclose acrylic copolymers for use in coating compositions include U.S. Pat. No. Re. 31,309 (to Antonelli et al.); U.S. Pat. No. 3,284,415 (to Horvath); U.S. Pat. No. 3,632,789 (to Wilhelm et al.); U.S. Pat. No. 3,773,710 (to Victorius); U.S. Pat. No. 3,846,368 (to Pettit); U.S. Pat. No. 3,919,351 (to Chang et al.); U.S. Pat. No. 3,998,768 (to Pettit); U.S. Pat. No. 4,404,248 (to Spinelli et al.); U.S. Pat. No. 4,415,697 (to Peng et al.); U.S. Pat. No. 4,330,458 (to Spinelli et al.); U.S. Pat. No. 4,649,045 (to Gaske et al.); and U.S. Pat. No. 4,670,512 (to McFadden). The acrylic copolymers disclosed in these patents, however, are significantly different from the present invention in composition, function, and/or application or end-use. These patents do not disclose--or suggest--moreover, the polyol blends of the present invention.